Optically active naphthalene compounds are known for use in liquid crystal materials, for example GB1603075 describes compounds of formula: ##STR4## where R* is optically active 2-methylbutyl. Other homologues of these described therein have liquid crystal characteristics, but are described only for use in nematic liquid crystal materials. Mol. Cryst. Liq. Cryst. Letters 4(3-4) p87-92 (January 1987) discloses the use of optically active naphthalene derivatives in ferroelectric smectic liquid crystals, but that publication is later than the priority data hereof.
Ferroelectric smectic liquid crystal materials use the ferroelectric properties of the chiral tilted smetic C, F, G, H, I and K phases (designated S.sub.C * etc, the asterisk denoting chirality). The S.sub.C phase is generally the most useful, as it is the most fluid, and it is particularly desirable that the material shows an S.sub.A or nematic (denoted N) phase at temperatures above the chiral smectic phase, to assist surface alignment in a liquid crystal device. Ferroelectric smectic liquid crystal materials desirably have low viscosity, S.sub.C phases that persist over a broad temperature range which includes ambient temperature, chemical and photochemical stability, and in particular have a high spontaneous polarisation coefficient, Ps, measured in nC cm.sup.-2. Such materials offer the possibility of very fast swiched liquid crystal display devices.
Although some single compounds show all of the desirable properties outlined above, ferroelectric smectic liquid crystal materials in use today generally consist of a mixture of two major components, each of which may itself by a single compound or a mixture of compounds, one component being termed a `host`, and the other being termed a `dopant`.
The host is generally a material that shows a smectic phase (preferably tilted smectic, especially S.sub.C) without being chiral. The dopant is or contains at least one optically active compound, without necessarily showing a smectic phase, although it is preferred if the dopant does itself show a smectic phase. The dopan when mixed with the host induces tilted smectic phases shown by the mixture to be chiral.
Preferably the dopant induces the mixture to show a high Ps, together with the other desirable properties mentioned above. Often tough a compromise is achieved, in which although the dopant induces a high Ps in the mixture, other desirable properties are to some extent sacrificed, for example the temperature at which the S.sub.C * phase disappears may be depressed.
It is an object of the present invention to provide novel dopants for use in ferroelectric smectic liquid crystal mixtures which induce a usefully high Ps in a mixture with a host, and which in part at least overcome the problems encountered by the use of known dopants.